A servomechanism of this kind has its output member actuated by a hydraulic actuator or the like. A change-over valve is mounted in the circuit for driving the actuator so that a displacing input applied to the input member may control the valve. The displacing output from the output member is fed back to the valve such that the position of the valve body of the change-over valve can be adjusted. In this way, the aforementioned augmented output can be produced which is proportional to the input.
In this prior art mechanism, the output displacement from the output member is fed back to the change-over valve by means of a linkage or the like. Therefore, the mechanism is made up of a large number of components and hence complex in structure. Further, such a structure is difficult to assemble. Also, it is difficult to make such a mechanism small and lightweight.